This invention is related to a photovoltaic element for directly converting an optical energy such as solar light into an electric energy and a method for manufacturing the same.
A heterojunction type photovoltaic element, in which an amorphous silicon layer or a micro crystalline silicon layer are deposited on a single crystalline silicon substrate, is well-known. The heterojunction can have its distinguish function when an impurity is doped on an amorphous silicon layer or a micro crystalline silicon layer.
In the amorphous silicon layer or mircocrystalline silicon layer which is doped, however, defects caused by doping increase and the characteristic of the heterojunction interface is degraded. The degradation of the interface characteristic results in a lower conversion efficiency because of a recombination of carriers in the case where these silicon layers are used for a photovoltaic element.
To overcome this problem, Japanese Patent Laid-Open No.70183/1991 (IPC:H01L 31/04) has proposed a photovoltaic element in which the heterojunction interface characteristic is improved by interposing a substantially intrinsic amorphous silicon layer between a single crystalline silicon substrate and an amorphous silicon layer for the purpose of decreasing defects at the interface.
In a conventional photovoltaic element, many uneven sections of line- or lattice-shape etc. are formed on a surface of a substrate by such processes as etching which uses resist, or anisotropic etching which employs alkaline solutions such as potassium hydroxide (KOH) or sodium hydroxide (NaOH) solutions or mechanical groove in order to improve short circuit current brought by the optical confinement effect.
FIG. 11 illustrates a structure of a photovoltaic element having the optical confinement which improves the heterojunction interface characteristic (hereinafter it is referred as an HIT structure). As shown in FIG. 11, an intrinsic amorphous silicon layer 2 is formed on an n-type crystalline silicon substrate 1 of which front surface has many uneven sections. A p-type amorphous silicon layer 3 is formed on the intrinsic amorphous silicon layer 2. A front electrode 4 is formed on the whole region of the p-type amorphous silicon layer 3 and a comb-like collecting electrode 5 is formed on the front electrode 4. A back electrode 6 is formed on the back surface of the substrate 1.
Although the comb-like collecting electrode 5 appears to be formed on the top of the pyramid-shape protruded section in FIG. 11, the actual width of the comb-like collecting electrode 5 is no less than 100 xcexcm. To help an understanding about the notion of the comb-like collecting electrode 5, the figure describes the electrode appears to be formed only on the top of the pyramid-shape protruded section. The actual comb-like collecting electrode 5 has a width equivalent to ten to twenty protruded sections of pyramid-shape.
In the above described conventional structure of the front surface of the substrate 1, a problem may occur when the intrinsic amorphous silicon layer 2 is formed on the substrate 1 by a plasma CVD method. When an amorphous semiconductor layer such as amorphous silicon is formed by a plasma CVD method, the thickness of amorphous semiconductor layer may not be uniform in the top a, the bottom b of the uneven section on the front surface, and the plain surface between a and b. As the thickness of the amorphous semiconductor film on the top a is thick and thin on the bottom b, particularly the amorphous semiconductor film may not be sufficiently deposited at the bottom b. In FIG. 11, the intrinsic amorphous silicon layer 2 and the p-type amorphous silicon layer 3 become thin at the bottom b, and it causes a lower open circuit voltage and short circuit between the electrode and the substrate, resulting in extremely degraded output characteristic of a photovoltaic element.
This invention has an objective to provide a photovoltaic element which solves the conventional problem as described above and improve an output characteristic and yields, and a method for manufacturing the same.
An amorphous or micro crystalline silicon layer on a crystalline silicon substrate having many uneven sections is formed on a photovoltaic element of the present invention, and bottoms of the uneven sections on the substrate are rounded.
When the bottom of the uneven section is rounded, the thickness of the amorphous or micro crystalline silicon layer which is formed thereon can be uniform.
An amorphous or micro crystalline silicon layer of different conductivity type on a front surface of a crystalline silicon substrate of one conductivity type having many uneven sections is formed on a photovoltaic element of the present invention, and bottoms of the uneven sections on the substrate are rounded.
A substantially intrinsic amorphous or micro crystalline silicon layer is preferably interposed between the front surface of the crystalline silicon substrate of one conductivity type and the amorphous or micro crystalline silicon layer of different conductivity type.
When the bottoms of the uneven sections are rounded, the thickness of the amorphous or micro crystalline silicon layer of different conductivity type which is formed thereon can be uniform. In particular, a open circuit voltage and fill factor of a photovoltaic element having an HIT structure which improves a characteristic of the heterojunction interface by interposing the substantially intrinsic amorphous or micro crystalline silicon layer. The substantially intrinsic amorphous or micro crystalline silicon layer reduces defects at the heterojunction interface with a crystalline silicon substrate and improves the characteristic of the heterojunction interface. Thus, the layer does not affect the improvement of the heterojunction interface even when dopant is diffused on the intrinsic amorphous or micro crystalline silicon layer in the subsequent processes.
The bottom is preferably formed so as to have a curved surface of a larger curvature than that of the top of the protruded section.
Furthermore, the bottom is preferably a curved surface of which radius is larger than 0.005 xcexcm, more preferably in the range 0.01-20 xcexcm.
A high doping layer of one conductivity type can be formed on the back surface of the crystalline silicon substrate of one conductivity type. By providing the high doping layer of one conductivity type, a BSF-type photovoltaic element can be obtained.
A high doping layer of one conductivity type containing an amorphous or micro crystalline silicon can be formed on the back surface of the crystalline silicon substrate. A substantially intrinsic amorphous or micro crystalline silicon layer is preferably interposed between the crystalline silicon substrate and the high doping layer of one conductivity type containing an amorphous or micro crystalline silicon layer.
By using this structure, a BSF type photovoltaic element can be obtained in a low temperature process. The substantially intrinsic amorphous or micro crystalline silicon layer can reduce defects at the heterojunction interface with a crystalline silicon substrate and improve the characteristic of heterojunction interface.
An amorphous or micro crystalline silicon layer of different conductivity type on a crystalline silicon substrate of one conductivity type having many uneven sections on both front and back surface of the substrate is formed on a photovoltaic element of the present invention, and bottoms of the uneven sections are rounded.
A substantially intrinsic amorphous or micro crystalline silicon layer is preferably interposed between the crystalline silicon substrate of one conductivity type and the amorphous or micro crystalline silicon layer of different conductivity type.
The bottom is preferably formed so as to have a curved surface of a larger curvature than that of the protruded section.
Furthermore, the bottom is preferably a curved surface of which radius is larger than 0.005 xcexcm, more preferably in the rage 0.01 to 20 xcexcm.
The bottom is preferably formed so as to have a curved surface of a larger curvature than that of the top of protruded section.
A transparent electrode is formed on the amorphous or micro crystalline silicon layer of different conductivity type, and a comb-like collecting electrode is formed on the transparent electrode. A transparent electrode is also formed on the back surface of the substrate, and a comb-like collecting electrode is formed on the transparent electrode.
By using this structure, a warp of the substrate is prevented even when the substrate is made thinner.
In a manufacturing method of a photovoltaic elements according to the present invention, many uneven sections are formed on a crystalline silicon substrate, a surface of the substrate is isotropically etched, and bottoms of the uneven sections on the front surface of the substrate are rounded. A substantially intrinsic amorphous or micro crystalline silicon layer is formed on the front surface of the substrate, and thereon an amorphous or micro crystalline silicon layer is formed.
A substantially intrinsic amorphous or micro crystalline silicon layer is formed by depositing an intrinsic amorphous or micro crystalline silicon layer by plasma resolution using raw material gas such as silane without mixing dopant gas when forming a layer. In the subsequent processes, dopant may be diffused in the substantially intrinsic or micro crystalline silicon layer. However, the substantially intrinsic amorphous or micro crystalline silicon layer is formed so as to avoid defects at the heterojunction interface with a single crystalline silicon substrate, and the interface characteristic is improved when the layer is formed without containing dopant gas. Thus, the interface characteristic is not affected even if dopant is diffused after forming a layer.
Wet etching using a mixed solution of hydrogen fluoride and nitric acid is preferably used as isotropic etching.
Many uneven sections on the crystalline silicon substrate can be formed in a first process of cleaning a surface of the crystalline silicon substrate and a second process of processing the crystalline silicon substrate surface in an alkaline solution to form uneven sections on the surface.
In the first process, an alkaline solution of the same kind as that in the second process is preferably used.
An alkaline solution containing an interface active agent is preferably used in the second process. In the above composition, the appearance of the surface after isotropically etched does not affect the strength of the short circuit current of a photovoltaic element since the bottoms of the uneven sections are slightly rounded. And the variety of the thickness of the amorphous or micro crystalline layer is prevented, eliminating the lowered open circuit voltage and short circuit between the substrate and the electrode. Therefore, it is possible to achieve an improved output characteristic of a photovoltaic element and high yields.